The present invention relates to a control system for a nuclear power generating system and more particularly to a control system for preventing unnecessary reactor power reductions.
The principles for the generation of power by a nuclear reactor have been well established and are well understood. Briefly, the reactor contains uranium or plutonium fuel elements in a core arrangement. Through the mechanism of neutron absorption and nuclear fission of the uranium or plutonium, large amounts of energy are released. This released energy manifests itself in the form of heat which is utilized to generate electricity. In the pressurized water reactor context, the heat is transferred to a primary coolant which continuously circulates through the core and carries the generated heat to a heat exchange boundary where a secondary coolant or working fluid is heated. Ordinarily the secondary coolant is water and is vaporized at the heat exchange boundary to produce steam. The steam is then circulated in a secondary system to a turbine for its ultimate use. The turbine is caused to turn at a predetermined rate and is connected to a generator for the ultimate transformation of the thermal energy to electrical energy.
All elements of this system are functionally interrelated. As an example, an increase in reactor power increases the rate of energy transferred to the primary coolant which in turn increases the rate of energy transferred to the secondary coolant causing more energy provided to the turbine for its ultimate transformation in electrical energy. Conversely, if less electrical energy is required, the energy requirement of a turbine diminishes. The steam flow to the turbine is reduced and consequently the turbine utilizes less of the thermal energy being transferred to the secondary coolant and an energy backup results. Since less energy is being drawn from the steam supply system when the steam flow is reduced, both the temperature and the pressure of the steam generator secondary side are caused to increase. The effect of this increase on the secondary coolant temperature is reflected on the primary side of the heat exchanger since less energy is being transferred across the heat exchange boundary. Accordingly, the primary coolant temperature and pressure increases.
Generally, most nuclear power generating systems are controlled such that reactor power level follows the turbine load. That is, changes in turbine load are sensed by the control system which in turn causes the reactor power level to be changed to be in agreement with the turbine load so that the energy generated by the reactor is equivalent to the energy utilized by the turbine. Normally, this is accomplished by the reactor regulating system which automatically drives control or regulating elements into or out of the reactor core in response to turbine load changes to change the power output of the reactor. It should be noted that most of the automatic control systems of typical nuclear power generating systems are only operable to provide automatic control of the reactor above a certain power level, generally 15% of a full power level output. Below this power level the plant is controlled manually. The reason for the low limit on automatic control is that system stability deteriorates with decreasing power level.
Generally, as noted herein above, if the reactor power is not reduced in response to a turbine load rejection or reduction, then serious increases may result in primary and secondary pressure and temperature. Such increases may also result even with the reactor regulating system operating to reduce the reactor power if the magnitude and rate of energy backup exceed certain values. This is a result of the fact that the regulating rods can only be advanced into and out of the core at a limited maximum speed.
Accordingly, changes in reactor power inherently occur at slower rates than can be imposed on the turbine. If the reactor power level is not reduced rapidly enough to compensate for the energy backup and the temperature and pressure of the primary system increases uncontrollably, then protective or control systems come into operation to trip the reactor and/or to open steam relief valves in order to avoid an overpressurization in the primary and secondary systems. If the uncontrolled increase in pressure is not avoided by these measures the safety pressure valves of either the primary or the secondary side are caused to lift. This is an undesirable occurrence since it may put the system out of operation until the seals of the safety valves have been remachined and reseated. Another undesirable effect is that the reactor is tripped unnecessarily upon a large or rapid load rejection that would otherwise not require taking the reactor out of operation. Such a trip temporarily removes the nuclear power plant as a supplier and a time consuming and expensive reactor startup procedure must be followed before the reactor can be put back into operation as a power producer.
Also, as can be appreciated, it may be undesirable to reduce the reactor power in response to a turbine load reduction if it is not absolutely necessary. For example, if the turbine load reduction is only temporary and the reactor power is reduced to follow the turbine, then when the turbine load is increased back to its original level, a substantial time lag will result before the reactor power can be brought back to its original level. Such a lag is due to the fact that reactor power changes are limited to much slower rates than those that the turbine is capable of accommodating.
Recently, systems have been developed which allow the reactor to operate at a higher power level than those which the turbine can handle without causing overpressurization of the primary and secondary systems. One such system is disclosed in co-pending U.S. patent application Ser. No. 347,260 filed Apr. 2, 1973 entitled "Steam Relief Valve Control System for a Nuclear Reactor" by Jose Marcelo Torres and assigned to the same assignee as the present application. That application discloses the use of steam relief valve operable in response to rises in secondary system pressure to bypass excess steam to the condenser or to dump excess steam to the outside atmosphere. Therefore, by using a plurality of bypass valves which are operable to prevent overpressurization in the primary and secondary systems, it is no longer absolutely necessary to reduce reactor power in response to turbine load reductions.